1. Field of the Invention
The present invention relates to a process for the preparation of (cyclo)aliphatic uretdiones having improved color quality.
2. Discussion of the Background
The preparation of (cyclo)aliphatic uretdiones is described in DE-A 37 39 549. They can be processed to give lightfast PUR powders. A disadvantage of the (cyclo)aliphatic uretdiones of DE-A 37 39 549 is their more or less severe discoloration. Surprisingly, it has been possible to largely eliminate such discoloration if a trivalent, phosphorus compound is present during the dimerization of the diisocyanate.
An object of the invention is to provide a process for the preparation of (cyclo)aliphatic uretdiones of improved color quality by reaction of (cyclo)aliphatic diisocyanates, if desired in an inert solvent, with the aid of from 0.5 to 3% by weight of a catalyst of formula (I) 
in the presence of from 0.1 to 4% by weight of a phosphorus compound of formula (II) 
The invention provides a process for the preparation of (cyclo)aliphatic uretdiones of improved color quality by dimerization reaction of (cyclo)aliphatic diisocyanates, if desired in an inert solvent, with the aid of from 0.5 to 3% by weight of a catalyst of formula (I) 
in which R1 and R2 are identical or different alkyl radicals of 1 to 8 carbon atoms or, together with the nitrogen atom to which they are attached, form a 5-membered or 6-membered ring which can contain a CHxe2x80x94CH3 group, a Nxe2x80x94CH3 group or an oxygen atom in place of one CH2 group in the ring,
in the presence of from 0.1 to 4% by weight of a phosphorus compound of formula (II) 
in which R3 and R4 are identical or different aromatic or araliphatic hydrocarbon radicals of 1 to 10 carbon atoms or are OR1 or OH. The dimerization is first carried out at temperatures of from 0 to 60xc2x0 C. up to a conversion of from 5 to 70% and then the 1,3-diazacyclobutane-2,4-dione formed is isolated as a residue, without prior deactivation of the catalyst, from the reaction mixture by thin-film distillation. The amine catalyst, phosphorus compound and unreacted diisocyanate are isolated from the reaction mixture by thin-film distillation as the distillate. German patent application 195 10 956.2 filed Mar. 25, 1995 is incorporated herein by reference in its entirety.
The catalysts are employed in quantities of from 0.5 to 3% by weight, preferably from 1 to 2% by weight. They are N,N-disubstituted 4-aminopyridine derivatives, such as 4-dimethylaminopyridine, 4-diethylaminopyridine, 4-pyrrolidinopyridine, 4-piperidinopyridine and 4-(4-methylpiperidino)pyridine.
The phosphorus compounds are trivalent phosphorus compounds such as, for example, triphenylphosphine, triethyl phosphite, tributylphosphite, dibutylphosphite and dibenzylphosphite. They are added in quantities of from 0.1 to 4% by weight, preferably from 0.5 to 2% by weight, to the diisocyanate to be dimerized. The (cyclo)aliphatic diisocyanates employed according to the process of the invention contain from 6 to 15 carbon atoms.
The process according to the invention is conducted such that, first of all, dimerization is carried out with the aid of an amino catalyst and a phosphorus compound described above, up to a conversion degree which still permits conveying the reaction mixture in the liquid state at room temperature. This ranges generally from 20 to 60% diisocyanate conversion, preferably 30 to 50%. Then the unreacted diisocyanate together with the catalyst and the phosphorus compound are separated from the reaction product by thin-film distillation. The diisocyanate distilled off (plus catalyst and phosphorus compound) can be recycled to the reaction.
The reaction temperature is within a range from 0 to 60xc2x0 C., preferably from 10 to 30xc2x0 C. At higher temperatures, the catalytic conversion of the uretdione to the corresponding isocyanurate is clearly-evident.
The reaction timexe2x80x94the time within which, for example, from 40 to 60% of the diisocyanate has reactedxe2x80x94depends (at constant temperature) to a great extent on the concentration and on the nature of the catalyst employed. It is generally from 10 to 90 hours. The reaction can be carried out in polar solvents, such as esters, ethers and ketones, or without solvent. It is preferably carried out without solvent.
The reaction mixture is worked up by thin-film distillation at from 100 to 180xc2x0 C. and from 0.01 to 0.5 mbar.
The uretdiones prepared by the process according to the invention are distinguished from the uretdiones of DE-A 37 39 549 by improved color quality.